The present invention relates to a connection accessory for connection of an electrical conductor such as an aluminum cable to a contact strip of an electrical apparatus such as a circuit breaker, and also to a terminal equipped with such an accessory.
Conventional connection terminals are designed to perform connection of copper cables to the contact strips of apparatuses such as circuit breakers, but are not suitable for connection of aluminum cables. This results in the user, when he wants to use aluminum cables, having to replace the kit formed by the tunnel and screw by another kit designed for connection of aluminum cables.
One of the reasons for this is that aluminum has a strong tendency to oxidize in the open air, which results in aluminum cables over the course of their use becoming covered with insulating alumina reducing the quality of the contact between the cable and contact strip.
The other reason is that aluminum presents a high expansion coefficient due to the effect of temperature variations. Circuit breakers, since they are supplied by variable currents, do however go through alternate heating and cooling phases. This results in temperature variations giving rise to variations in the dimensions of the cables by differential expansion. The cables are, however, in general in contact on the one hand with the contact strip most frequently made of copper, and on the other hand either with the steel screw or with the tunnel also made of steel. But these three materials present different expansion coefficients, which results in them not retracting in the same manner during the cooling phases. This leads to a connection loosening phenomenon which on the one hand, in itself, reduces the quality of the electrical contact, and on the other hand fosters formation of a layer of alumina also liable to reduce the quality of the electrical contact.
Several solutions have been imagined to overcome these shortcomings.
One of the solutions is described in the German Patent DE 4,026,037. In this document, the cable is brought into contact with the contact strip by a steel tunnel moved in translation by the terminal screw. In order to overcome the problem of oxidation of the aluminum, the copper contact strip and the aluminum tunnel respectively present two serrations facing one another forming elongate claws designed to pass through the layer of alumina covering the cable, so as to improve the contact between the contact strip and tunnel.
In this document, one of the above-mentioned problems has therefore been solved by achieving a specific system for connection of aluminum cables, which due to the presence of these claws presents a high cost price.
Another solution is known wherein the terminal is formed by an aluminum block comprising on the one hand an elongate passage for passage of the contact strip and on the other hand a threaded orifice designed for passage of the terminal screw also made of aluminum. In this embodiment, the bottom wall of the tunnel presents a U-shaped cross section so as to match the shape of the cable, and the screw presents a convex end so that the cable is held captive between the bottom of the tunnel and the screw. In this embodiment, all the component parts being made of aluminum, no differential expansion occurs. The absence of differential expansion of the different elements and the fact that the cable is closely surrounded enable the problem of expansion of the aluminum cables and of formation of alumina on the cable to be satisfactorily solved. However, this solution still presents certain drawbacks resulting on the one hand from the fact that this tunnel has to present a large thickness whence a large quantity of alloy being necessary, and on the other hand from the fact that the tunnel being liable to oxidize itself needs to be tinned. This results in a terminal which is delicate to manufacture, costly and bulky.